In testing within clinical laboratories to measure various chemical constituents of body fluids obtained from patients, such as whole blood, blood serum, blood plasma, and the like, fully-automated clinical analyzers may reduce the number of trained technicians required to perform the analyses, improve accuracy of the testing, and reduce the cost per test.
Typically, an automated analyzer includes an automated aspirating and dispensing apparatus, which is adapted to aspirate a liquid (e.g., a sample of biological liquid or a liquid reagent) from a container and dispense the liquid into a reaction vessel (e.g., a cuvette). The aspirating and dispensing apparatus typically includes a pipette (otherwise referred to herein as a “probe”) mounted on a moveable arm or other automated mechanism, to perform the aspiration and dispensing functions and transfer the sample to the reaction vessel.
During the aspiration operation, the moveable arm, which may be under the control of a robotic controller, may position the probe above the container, and descend the probe into the container until the probe is partially immersed in the liquid (e.g., biological liquid sample or liquid reagent) in the container. A pump or other aspirating device is then activated to draw in (aspirate) a portion of the liquid from the container into the interior of the probe. The probe is then ascended (retracted) from the container such that the liquid may be transferred to the reaction vessel for testing. During or after the aspiration, an aspiration pressure signal may be analyzed to determine any anomalies, i.e., check for the presence of a clog or the presence of air should there be insufficient liquid remaining to carry out the desired aspiration.
Conventional systems are able to perform these checks acceptably when relatively large liquid volumes are aspirated (e.g., 30 μL or greater). However, if the volume of the aspirated sample is relatively small, the noise in the pressure signal becomes so large that it may obscure the information contained in the pressure reading. Accordingly, when the aspirated liquid volume is relatively small, it may become very difficult to robustly determine the difference between an air aspiration, i.e., where no liquid (e.g., biological liquid sample or liquid reagent) is aspirated, and a proper aspiration. Similarly, if a clot or other undesirable material were aspirated, it becomes challenging to determine the difference between the aspiration of such clot or other undesirable material and proper liquid sample aspiration at such small volume aspiration. Accordingly, there is a need for a method and apparatus to accurately determine aspiration pressure when the aspirated volume of the liquid is relatively small so that an adequate check may be carried out.